The deterioration of the humeral head and the glenoid by primary or secondary osteoarthritis causes a loss of articular congruence, which is a source of pain and limits the mobility of the shoulder.
One solution to offset this joint deterioration consists of implementing a complete shoulder prosthesis including a humeral head designed to be engaged in the medullary channel of the humerus, and a glenoid implant designed to be implanted in the glenoidal cavity of the scapula. Such a complete shoulder prosthesis may be anatomical or said to be reversed when the convexity of the glenoid is transferred to the location of the humeral head.
The fastening of the glenoid implant in a glenoidal cavity requires producing one or more bone drillings in the latter designed to house the fastening element(s) provided on the glenoid implant, such as a fin or fastening studs.
Because of the anatomy of the glenoid and the scapula and the low bone capital of the bony vault of the glenoid, the production of such bone drillings must be done precisely so as to limit the risks of perforation of the bony vault of the glenoid and therefore the risks of subsequent conflicts between the fastening element(s) of the glenoid implant and the anterior or posterior cortex of the scapula. Such conflicts may harm the stability of the final implant and impose removal of the shoulder prosthesis. Furthermore, protrusion of the fastening elements outside the bone would cause friction of the latter on the muscles or nerves (suprascapular), which may cause pain for the patient.
In order to limit the risks of such conflicts, it is known to use a device for guiding piercing tools as described in document U.S. Pat. No. 5,437,677.
The guide device described in document U.S. Pat. No. 5,437,677 comprises:                a support plate provided with an indexing member including a bearing portion designed to bear on the anterior cortex of the scapula,        a guide body comprising a bearing surface designed to bear in a previously prepared glenoidal cavity of the scapula, and a through orifice emerging in the bearing surface, the guide body being slidingly mounted on the support plate in a movement direction parallel to the axis of the through orifice, and        means for immobilizing the guide body with respect to the support plate.        
In order to prepare the glenoidal cavity for the placement of the glenoid implant, the surgeon positions the bearing portion of the reference member against the anterior cortex of the scapula, then moves the guide body toward the glenoidal cavity until the bearing surface of the guide body bears against the glenoidal cavity, and then immobilizes the guide body with respect to the support plate using immobilizing means. Lastly, the surgeon inserts a piercing tool through the through orifice of the guide body and produces a bone drilling in the glenoidal cavity.
It should be noted that the distance separating the axis of the through orifice and the bearing portion is defined such that, when the bearing portion of the reference member is in contact with the anterior cortex of the scapula, the axis of the through orifice extends in the central part of the glenoid neck.
Such a guiding device consequently makes it possible to greatly limit the risks of perforation of the bony vault of the glenoid, and therefore the risks of removal the shoulder prosthesis by pulling free of the glenoid.
However, such a guide device may be unsuitable for implanting glenoid implants in patients with a particular anatomy or significant deteriorations of the glenoid. In fact, in these particular cases, the definition of the distance separating the axis of the through orifice and the bearing portion may be such that the bone drilling done will extend in an area of the glenoid having poor bone quality or will emerge in the anterior cortex of the scapula.